From When Harry Met Sally (1989). For those who haven't seen it, the story follows two friends, Harry (Billy Crystal) and Sally (Meg Ryan), over several years and various relationships. Harry claims that men and women can never be friends, while Meg argues that they can. To find out who is right, you'll have to see the movie. At one point Harry and Sally argue about fake orgasms, which leads to this:

Any more nominees for the best or worst fake orgasm scene ever (excluding porn - that'd be too easy)?